The present invention pertains to a plasma reactor which utilizes electromagnets to enhance plasma.
Plasma etching is a well known dry etching technique which physically and chemically etches a workpiece. The basic plasma reactor consists of an evacuated chamber with electrodes. An etching gas, such as CF.sub.4, is introduced into the chamber and is disassociated by the flow of electrons between electrodes, to form a plasma. One of the electrodes, acts as a stage for the workpiece. The workpiece, is then physically and chemically etched by the plasma. The disassociation of the etching gas produces ions with high kinetic energy which bombard the workpiece thereby physically etching it. The disassociation of the etching gas also produces reactive neutrals which react with the workpiece thus chemically etching it.
It has been found that relatively high pressure plasma etchers achieve high etch rates from increased reactive radical densities due to the decreased collisional mean free path of electrons in the discharge. This however, results in an increased degree of isotrophy.
Anisotropic etching can be obtained by decreasing the pressure in the reaction chamber. In this case anisotropy is increased but etch rate is lowered. The lowering of the etch rate is due to the decreased production of reactive radicals due to the increased collisional mean free path of electrons in the discharge.
A low pressure plasma etcher with high etch rates can be obtained by utilizing magnets. A magnetic field perpendicular to the electric field causes electrons to execute a cycloidal motion which continues until the electron suffers an exciting or ionizing collision with a gas molecule producing reactive radicals. While anisotropic etching and increased etch rates are achieved uniformity suffers due to the concentration of reactive radicals along magnetic field lines.
In U.S. Pat. No. 4,526,643 titled "Dry Etching Apparatus Using Reactive Ions", issued July 2, 1985, a dry etching apparatus employing permanent magnets is disclosed. These magnets are attached to a belt which is rotated underneath the workpiece. This causes a scanning motion of the magnetic field across the surface of the workpiece. In bar magnets the magnetic lines connecting north and south poles are curved. The bigger the bar magnet, the less the magnetic lines will be curved. In order to approximate a linear magnetic line, the magnet must be very large. However, even the magnetic lines of large magnets will not become completely linear. Thus, in this prior art with the magnets being necessarily limited in size due to the number, space and mechanics involved, the field is non-linear and results in reduction in uniformity of the etch. Further, the mechanical movement of the magnets themselves is very difficult when located inside a vacuum and can cause numerous problems.